Coating, adhesive and sealing compositions which are to be applied to vertical or non-horizontal surfaces desirably have certain limited flow properties. Advantageously, such compositions should remain essentially in the area to which they have been applied until they have had sufficient time to harden and acquire there final shape. This is especially important for compositions which contain one or more liquid components or reactants. Exemplary of such are polyurethane sealant foam compositions comprising a liquid polyisocyanate and a liquid polyahl component. The polyisocyanate and polyahl should be sufficiently mobile to permit their intimate mixing and yet once mixed the reacting mass should not be so mobile that it can flow away from the region to which it has just been applied. To this end, a thixotropic agent is usually present in the composition. A thixotropic agent is a substance which can impart to a composition a liquid state when such composition is shaken or stirred and a gel like state to the composition when not physically agitated.
Frequently employed thixotropic agents include finely divided particles or solids such as fumed silica, polyurethane scrap powder or organic polymers such as polyureas obtainable from aliphatic or aromatic polyamines and aliphatic or aromatic polyisocyanates. French patent 2,447,955 discloses a multipurpose thixotropic mastic coating composition containing volcanic slag, polyurethane scrap powder and a polyester resin. Japanese patent 57-195,765 discloses thixotropic polyurethane compositions obtainable by mixing an isocyanate-terminated prepolymer with an aromatic carboxylic acid ester in the presence of carbon black. U.S. Pat. No. 4,438,235 discloses the use of tertiary alkanolamines as thickening additive for polyols having a molecular weight of from 100 to 2000. Japanese patent 63-015876 discloses a urethane sealant composition which comprises an isocyanate-terminated prepolymer, thixotropic fatty acids, filler and plasticizer. U.S. Pat. No. 4,801,623 discloses the preparation of a thixotropic agent for storage stable polyurethane systems from a uretdione group-containing polyisocyanate with a polyfunctional carboxylic acid and a high molecular weight alcohol. Other patent publications describing the use of thixotropic or thickening agents for polyurethanes include U.S. Pat. Nos. 5,023,309; 3,923,926; 4,945,128; J02-167365; J01-014269 and EP-129,858.
The above-mentioned thixotropic systems and thickening additives in general fulfill the requirements of permitting the preparation of polyurethane foam at a non-horizontal application area. However in many instances the resulting foam is observed to have an inadequate dimensional stability with often severe foam shrinkage occurring in a relatively short period of time. This problem can be particularly acute when the physical blowing agent normally present in the foaming process is partially or totally replaced by water. Use of water is considered highly desirable in view of current opinions over the stability of the atmospheric ozone layer and the alleged association of certain per(chlorofluoro)carbons with its depletion. An additional undesirable feature observed when attempting to prepare such water blown polyurethane foam is that the dimensional stability of the resulting foam is influenced by the time for which the polyahl/thixotropic agent composition has been stored prior to foaming. Extended storage times are observed to frequently result in foam with inferior dimensional stability.
It would be therefore desirable to develop an alternative additive for use when preparing polyurethane foam, especially rigid polyurethane foam on a non horizontal surface, which can permit the use of significant amounts of water in a foaming process and provide for foam which has an enhanced dimensional stability. It would be also to advantage if such additive, when formulated with the polyahl component, could allow for an extended storage period of the resulting composition with a minimum detriment to the dimensional stability characteristics of the foam eventually prepared therefrom.